25GBASE-SR transceivers<\/a>, whose standardized reach is typically 70 meters over OM3 and 100 meters over OM4, AOC reach is determined by the specific cable design and vendor qualification. Commercial 25G SFP28 AOCs are commonly available from a few meters to around 30 meters, with some implementations supporting longer distances.<\/p>\n\n\n\nAOC also provides excellent immunity to electromagnetic interference because the signal travels through fiber as light rather than as an electrical signal.<\/p>\n\n\n\n
The trade-offs are higher power consumption and cost compared with passive DAC. An AOC commonly consumes around 1 watt per connector end, although actual values vary by vendor and design. Optical conversion electronics can also add some device latency, but total link latency depends on cable length, PHY implementation, and FEC configuration. Fiber propagation itself contributes roughly 5 nanoseconds per meter, while FEC processing can contribute substantially more latency than the optical conversion alone.<\/p>\n\n\n\n
In exchange, you get a thin, lightweight, flexible assembly that routes cleanly through dense racks and performs predictably in electrically noisy environments.<\/p>\n\n\n\n<\/span>SFP28 DAC vs AOC: Side-by-Side Comparison<\/strong><\/strong><\/span><\/h2>\n\n\n\nHere’s the table most engineers actually keep open while ordering.<\/p>\n\n\n\nFeature<\/strong><\/td>SFP28 Passive DAC<\/strong><\/td>SFP28 Active DAC (ACC)<\/strong><\/td>SFP28 AOC<\/strong><\/td><\/tr>Transmission medium<\/td> Copper twinax<\/td> Copper twinax<\/td> Multimode fiber (OM3\/OM4)<\/td><\/tr> Typical reach<\/td> 0.5-5 m<\/td> 5-10 m<\/td> 25-100 m (70 m OM3 \/ 100 m OM4)<\/td><\/tr> Power per link<\/td> ~0-0.1 W<\/td> ~0.5-1 W<\/td> ~1 W per module<\/td><\/tr> Latency<\/td> <0.1 \u00b5s<\/td> ~0.1-0.2 \u00b5s<\/td> ~0.3-0.5 \u00b5s<\/td><\/tr> Relative cost<\/td> Lowest<\/td> Moderate<\/td> Highest<\/td><\/tr> EMI immunity<\/td> Susceptible<\/td> Susceptible<\/td> Immune<\/td><\/tr> Cable bulk<\/td> Thick, heavy<\/td> Thick, heavy<\/td> Thin, light, flexible<\/td><\/tr> Bend radius<\/td> Large<\/td> Large<\/td> Tight<\/td><\/tr> Electronics<\/td> None<\/td> Linear equalizer chip<\/td> Integrated optical transceivers<\/td><\/tr> Best for<\/td> Same-rack ToR links<\/td> Adjacent-rack, longer copper runs<\/td> Cross-rack, row-to-row, high-EMI sites<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nThe headline is simple: DAC wins on cost, power, and latency for short runs; AOC wins on distance, flexibility, and EMI immunity. The nuance is where those thresholds actually sit.<\/p>\n\n\n\n
<\/span>When to Choose SFP28 DAC Over AOC<\/strong><\/strong><\/span><\/h2>\n\n\n\nStart with DAC for anything inside the same rack or between adjacent racks. A server to top-of-rack switch link is usually 1 to 3 meters, well inside passive DAC territory. You get the lowest price per port, near-zero power draw, and the lowest possible latency, which matters for HPC and financial trading fabrics.<\/p>\n\n\n\n
Rani, a network engineer in Chicago, spent a month planning a 144-server leaf deployment. Her first instinct was to buy AOC everywhere “to be safe.” The quote came back roughly $6,000 higher than the equivalent passive DAC solution, for links that were all under 3 meters. She switched to passive DAC for every in-rack link and reserved AOC for the handful of runs crossing between racks. The deployment came in under budget and nothing ever failed to link.<\/p>\n\n\n\n
Use passive DAC when your link is 5 meters or less. Step up to active DAC (ACC) when the path runs 5 to 10 meters, or when copper routing through tight cable trays is the constraint. Active DAC gives you copper’s cost and latency profile with enough reach to span adjacent racks.<\/p>\n\n\n\n
<\/span>When to Choose SFP28 AOC<\/strong><\/strong><\/span><\/h2>\n\n\n\nReach beyond 10 meters is the clearest trigger for AOC. A cross-aisle run from one rack row to another is typically 20 to 50 meters, and no copper assembly at 25G survives that distance without active electronics on every hop.<\/p>\n\n\n\n
AOC also wins in specific environments regardless of distance. If the cable path passes near power distribution, industrial equipment, or anything that radiates EMI, fiber’s immunity is worth the premium. If you’re packing high port density into leaf switches and every millimeter of airflow matters, the thin flexible AOC assembly is dramatically easier to route than stiff copper bundles.<\/p>\n\n\n\n
Mei, a facilities lead in a shared co-location building, found this out the hard way. Her team ran passive DAC from a ToR switch to servers, but the shortest cable path crossed a row of power strips feeding the neighboring tenant’s racks. The copper links flapped randomly for weeks.<\/p>\n\n\n\n
Moving to AOC on those specific runs eliminated the problem entirely. The cables cost more. The stability was worth every dollar.<\/p>\n\n\n\n
The practical rule: DAC inside the rack, AOC between racks. Most 25G data centers run a hybrid mix, and you should too.<\/p>\n\n\n\n
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